There are multiple breaking reports that some kind of lockdown is occurring in Paris.

Here are the latest reports from Twitter users who are in the area:

Notre Dame in Paris Lockdown

Matthew CurrieHolmes writes:

We're trapped in Notre-Dame de Paris, something is happening outside. Police sirens can be heard. They are not letting anyone in or out...We were just told that the doors will remain locked until the police tell us otherwise. We have no idea what's happening...Reports indicate that a gunman was out front the police have shot him. I cannot confirm this b/cme and my family are stuck in the church

...

Apparently someone attacked a police officer with a hammer and the police officer shot and killed him.

While there are reports of a gunman, most of the reports are saying that the man had a hammer and that he was trying to attack the police.

It appears that there may be an injury but there is currently no evidence that anyone has been killed. The incident appears to have occurred outside of the cathedral.

The police officer appears to have shot and injured the attacker:

MORE: Paris police confirm officer injured by a man who was shot and injured by police.